Valid score (Ippon) - without hitting the target?

Yes.

Is the short answer.

Of course it is absolutely ideal if the shinai hit bang on the target and the flags go up, but it is also quite common that a player is awarded an Ippon even though their shinai is clearly not hitting the target.

Why?

It is down to "how convincing" their Spirit-Sword-Posture (Ki-Ken-Tai) was in the right moment of a given combating narrative.

The referee will award an Ippon for the demonstration of the prowess in the tactical judgement & execution of the technique, (crucially) in the *given* combating context.

The idea is that, the player don't need to provide a physical evidence (hitting) in a close match, but the one who can demonstrate their prowess better than their opponent will get an Ippon.

If a strike is launched in the right context with convincing enough power and form, that counts as an Ippon, even if the shinai is not physically touching the target.

Hence the same strike with different opponent - or even with the same opponent in a different combating scenario - may not bring the same result.

And remember, this criteria applies to everyone; this may benefit your opponent one time but it might benefit you in another.

The best (and perhaps the only) strategy to avoid such a 'phantom-Ippon' taken from you, is to make your Kendo totally convincing.

The phantom-Ippon occurs only when your performance is not convincing enough in contrast to your opponent. Maybe it's the strategy or the timing, or poor Zan-shin. So never take it out on the referee.

Yes, there are inexperienced referees, but they have to start from somewhere, also you might be refereeing one day and other people may not agree with you. What goes around, comes around. We're all leaning.

If you cannot convince inexperienced referees, you just got to make your Kendo totally unarguable even for the inexperienced referees, that's all.

So, no "booing" please during the up coming 16th World Kendo Championship.

Such unsophisticated behaviour should remain inside you. It's OK to feel the high spirit, but you really don't need to share it with people around you as Kendo is not a spectator sports (though often misunderstood so).

If a match developed in a certain way, players and the audience should calmly take it as is. That's part of self-control.

If simply 'hitting' is paramount, that's a stick fight which is nothing to do with the swordmanship that Kendo is aiming to promote.

Yes, it might feel unfair sometimes, but you need to be disciplined to take it on the chin and focus on making your Kendo totally convincing.